This work investigates the microstructural features and mechanical properties of joints of lowcarbon steel friction stir spot welded by changing tool rotational speed and dwell time. Microstructural evaluation, peeling, tension-shear and micro-hardness tests were employed to characterize the properties of the welded joints. The results show that an increase in the tool rotational speed promotes the formation of more weld flash, and it equally changes the flash morphology from ring flash to a combination of ring and serrated flashes. A fine immediate tool contact region is formed in the stir zone (SZ) and the width of the ITCR increases with the tool rotational speed and tensile-shear failure load. Besides the ITCR, the SZ uniquely consists of the shoulder undersurface region (SZ I) and vortex region (SZ II) with average grain sizes of about 10.9 and 16.66 lm, respectively. An increase in tool rotational speed improves the micro-hardness, and the average diameter of the weld nugget (from 4.5 to 5.5 mm). Base metal–nugget interface failure (650 rpm) and base metal neck inginduced failure (1250 rpm) are the two forms of weld failure obtained in the welded samples.